Influenza (flu) is a viral illness that can range from mild to severe and primarily affects the respiratory system. In certain groups of people, the very young and the elderly, it can cause serious complications. These complications often lead to hospitalization in the elderly and very young population and may lead to death.

Key Facts:

  • Influenza is a contagious respiratory illness.
  • People infected with the flu can spread the disease up to 1 day before symptoms are present and up to 5-7 after becoming ill.
  • The circulating influenza virus is always changing and a new vaccine is required before every flu season.

Did You Know?

The 1918 Influenza Pandemic (Spanish Flu) which killed between 21-50 MILLION people most likely started in the US. It was the first of the H1N1 pandemics. While many medical advances were made in trying to find the infectious agent, scientists would not even identify the influenza virus until the 1930’s.

What are the symptoms of influenza?

  • Fever/chills (occurs with most people but not everyone)
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Muscle or body aches
  • Headaches
  • Fatigue
  • Some people may have vomiting and diarrhea, though this is more common in young children than in adults.

(Taken from CDC Key Facts About Influenza)

Flu-like Illness

There are many different viruses and other “germs” that can cause us to feel like we have the flu. They may cause us to have the same symptoms such as fever, body aches, coughing, headache and malaise. It is difficult to track actual influenza incidence so the CDC tracks flu like illnesses. However, an infection with influenza A can be much more serious in people than some of the other viruses circulating in winter, such as rhinovirus, which is to blame for most common colds.

In addition, true influenza is different than the “stomach flu” which is usually caused by viruses such as rotavirus, norovirus or adenovirus. There are, of course, many other pathogens that cause gastroenteritis and flu-like illnesses.

The Influenza Vaccine

Let’s start with the most common question: Why do we need the influenza vaccine every year?

First, we need to define the antigen. An antigen is any substance (such as bacteria, viruses, or chemicals) foreign to the body that evokes an immune response either alone or in conjunction with a larger molecule that is capable of binding with a product (such as an antibody or T cell) of the immune response. (check out Merriam-Webster: they had the clearest definition)

Re-vaccination with the flu shot annually is due to a concept in virology called “antigenic drift.” Antigenic drift is the very small changes or mutations in the genes of the virus. Over time, these minute changes add up and the body is no longer able to recognize the virus rendering the previously developed antibodies in-effective. Meaning, the body no longer mounts an appropriate immune response. This applies to antibodies that are a result of immunization or those from being previously infected with the disease.

Something much more serious than “antigenic drift” is “antigenic shift,” which is an abrupt and significant change in the antigen. Antigenic shift occurs when the influenza A virus, which is normally found in birds*, begins to infect humans. Because the virus is so different from the more common circulating influenza viruses, humans have no immunity to it. This lack of immunity allows the virus to spread rapidly through a population setting the stage for a pandemic. Pandemics occur about 3 times in a century. Our last pandemic was the H1N1 influenza in 2009.

Antigenic shift is especially dangerous because the vaccine in development or current use will most likely not protect people. Rapid development of a new vaccine is then required, and while public health anticipates that scenario, it does take time.

*Birds are the primary carriers of influenza and can transmit the infection to people on occasion. It is only when the virus mutates again and allows the infection to transmit from person to person does public health concern rise significantly. This is why the WHO tracks bird to human transmission closely.

The influenza virus is an incredibly complex virus. It would be difficult to explain it here. A good book is The Great Influenza by John M. Barry (2004) for basic information on not just the influenza virus, but it is also a fascinating read on the 1918 influenza pandemic.

Possible Side Effects of Influenza Vaccine

A vaccine, like any medicine, is capable of causing serious problems, such as severe allergic reactions. Getting the influenza vaccine is much safer than getting the disease.

Adverse reactions to the flu vaccine may include:

  • Adverse reactions to the vaccine include:
  • Soreness, redness, and/or swelling from the shot
  • Headache
  • Fever
  • Nausea
  • Muscle aches

If these problems occur, they usually last 1 or 2 days.

What About Guillain-Barre Syndrome?

Guillain-Barre syndrome (GBS) occurs in approx. 1-2 people of every 100,000 and is more likely to occur in adults. The exact causes of GBS are unknown but one common factor seems to be a recent illness or stimulation of the immune system. That includes being infected with the flu virus. Because GBS is so rare, finding a casual pattern is difficult. However, some studies suggest it may occur more often after a flu illness than in those who received the vaccine.

The perceived association between GBS and the influenza comes from the 1976 influenza vaccine program. Anticipating a high number of swine flu cases that year, Congress allocated funds for a massive vaccination rollout. Shortly after the program began, a slight increase in cases of GBS were reported, approximately 1 additional case for every 100,000 people. The pandemic never materialized but trust was lost. Additional studies (2 in total) have found that approximately 1 additional case of GBS may occur for every 1 million vaccinated. While GBS is a terrible disease with recovery lasting weeks or years, the risk of being hospitalized or dying from influenza is much higher.